Thyroid Hormone Regulation Pituitary Thyrotropes TRH (CRH) Hypothalamus Thyroid Gland Transthyretin (-) T4 Iodine TPO DIT T4 TSH Peripheral Tissue T3 + TR/RXR DNA mRNA Liver T3 Deiodination (D2) T4 Deiodination (D3) Conjugation Inactivation/ Elimination Deiodination (D2) T4 NIS MITDIT Thyroglobulin colloid Follicular cells